Conditions affecting the proper functioning of the mitral valve include, for example, mitral valve regurgitation, mitral valve prolapse and mitral valve stenosis. Mitral valve regurgitation is a disorder of the heart in which the leaflets of the mitral valve fail to coapt into apposition at peak contraction pressures, resulting in abnormal leaking of blood from the left ventricle into the left atrium. There are a number of structural factors that may affect the proper closure of the mitral valve leaflets. For example, many patients suffering from heart disease experience dilation of the heart muscle, resulting in an enlarged mitral annulus. Enlargement of the mitral annulus makes it difficult for the leaflets to coapt during systole. A stretch or tear in the chordae tendineae, the tendons connecting the papillary muscles to the inferior side of the mitral valve leaflets, may also affect proper closure of the mitral annulus. A ruptured chordae tendineae, for example, may cause a valve leaflet to prolapse into the left atrium due to inadequate tension on the leaflet. Abnormal backflow can also occur when the functioning of the papillary muscles is compromised, for example, due to ischemia. As the left ventricle contracts during systole, the affected papillary muscles do not contract sufficiently to effect proper closure.
Mitral valve prolapse, or when the mitral leaflets bulge abnormally up in to the left atrium, causes irregular behavior of the mitral valve and may also lead to mitral valve regurgitation. Normal functioning of the mitral valve may also be affected by mitral valve stenosis, or a narrowing of the mitral valve orifice, which causes impedance of filling of the left ventricle in diastole.
Typically, treatment for mitral valve regurgitation has involved the application of diuretics and/or vasodilators to reduce the amount of blood flowing back into the left atrium. Other procedures have involved surgical approaches (open and intravascular) for either the repair or replacement of the valve. For example, typical repair approaches have involved cinching or resecting portions of the dilated annulus.
Cinching of the annulus has been accomplished by the implantation of annular or pen-annular rings which are generally secured to the annulus or surrounding tissue. Other repair procedures have also involved suturing or clipping of the valve leaflets into partial apposition with one another.
Alternatively, more invasive procedures have involved the replacement of the entire valve itself where mechanical valves or biological tissue are implanted into the heart in place of the mitral valve. These are conventionally done through large open thoracotomies and are thus very painful, have significant morbidity, and require long recovery periods.
However, with many repair and replacement procedures the durability of the devices or improper sizing of annuloplasty rings or replacement valves may result in additional problems for the patient. Moreover, many of the repair procedures are highly dependent upon the skill of the cardiac surgeon where poorly or inaccurately placed sutures may affect the success of procedures.
Less invasive approaches to aortic valve replacement have been developed in recent years. Examples of pre-assembled, percutaneous prosthetic valves include, e.g., the CoreValve Revalving® System from Medtronic/Corevalve Inc. (Irvine, Calif., USA) the Edwards-Sapien® Valve from Edwards Lifesciences (Irvine, Calif., USA). Both valve systems include an expandable frame housing a tri-leaflet bioprosthetic valve. The frame is expanded to fit the substantially symmetric circular aortic valve. This gives the expandable frame in the delivery configuration a symmetric, circular shape at the aortic valve annulus, perfectly functional to support a tri-leaflet prosthetic valve (which requires such symmetry for proper coaptation of the prosthetic leaflets). Thus, aortic valve anatomy lends itself to an expandable frame housing a replacement valve since the aortic valve anatomy is substantially uniform and symmetric. The mitral valve, on the other hand, is generally D-shaped and not symmetric, meaning that expansion of the CoreValve and Sapien systems in the mitral valve renders such systems non-functional. For example, in both systems the frame both anchors (or helps to anchor) and provides shape to the replacement valve within. If the frame is flexible enough to assume the asymmetric shape of the mitral valve, then the attached tri-leaflet replacement valve will also be similarly shaped, making it almost impossible for the leaflets to coapt properly and thus allowing leaks. Additionally, if the frame is so rigid that it remains symmetric, the outer diameter of the frame will not be able to cover the commissures of the mitral valve, also allowing leaks.
In addition, mitral valve replacement, compared with aortic valve replacement, poses unique anatomical obstacles, rendering percutaneous mitral valve replacement significantly more involved and challenging than aortic valve replacement. First, unlike the relatively symmetric and uniform aortic valve, the mitral valve annulus has a non-circular D-shape or kidney-like shape and may be of unpredictable geometry, often times lacking symmetry. Such unpredictability makes it difficult to design a mitral valve prosthesis having the ability to conform to the mitral annulus. Lack of a snug fit between the leaflets and/or annulus and the prosthesis leaves gaps therein, creating backflow of blood through these gaps. Placement of a cylindrical valve prosthesis, for example, may leave gaps in commissural regions of the native valve, potentially resulting in perivalvular leaks in those regions.
Current devices seeking to overcome the large and irregular shape of the mitral valve have several drawbacks. First, many of the devices today have a direct, structural connection between the device structure which contacts the annulus and/or leaflets and the device structure which supports the prosthetic valve. In several devices, the same stent posts which support the prosthetic valve also contact subannular tissue, directly transferring many of the distorting forces present in the heart, for example, systolic pressure, diastolic pressure, compressive intra-annular forces, etc., causing hoop stress in the stent portion surrounding the prosthetic valve. Most cardiac replacement devices utilize a tri-leaflet valve, which requires a substantially symmetric, cylindrical support around the prosthetic valve for proper opening and closing of the three leaflets. Devices which provide a direct, mechanical connection between annular and/or leaflet distorting forces and the prosthetic valve may compress and/or distort the symmetrical, cylindrical structure surrounding the prosthetic valve causing the prosthetic leaflets to malfunction.
In addition to its irregular, unpredictable shape, the mitral valve annulus lacks a significant amount of radial support from surrounding tissue. The aortic valve, for example, is completely surrounded by fibro-elastic tissue, helping to anchor a prosthetic valve by providing native structural support. The mitral valve, on the other hand, is bound by muscular tissue on the outer wall only. The inner wall of the mitral valve is bound by a thin vessel wall separating the mitral valve annulus from the inferior portion of the aortic outflow tract. As a result, significant radial forces on the mitral annulus, such as that imparted by expanding stent prostheses, could lead to collapse of the inferior portion of the aortic tract with potentially fatal consequences.
The chordae tendineae of the left ventricle may also present an obstacle in deploying a mitral valve prosthesis. This is unique to the mitral valve since aortic valve anatomy does not include chordae. The maze of chordae in the left ventricle makes navigating and positioning a deployment catheter more difficult in mitral valve replacement and repair. Deployment and positioning of a prosthetic valve or anchoring device on the ventricular side of the native valve is also complicated by the presence of the chordae.
Given the difficulties associated with current procedures, there remains the need for simple, effective, and less invasive devices and methods for treating dysfunctional heart valves.